As the Internet becomes widespread in recent years, optical transmission technology for transmitting large-capacity signals over a long distance is desired and optical transmission technology of a WDM system comes into wide use. As transmission capacity of respective wavelengths of a WDM signal becomes large, a large amount of data is damaged even owing to a failure that occurs on one wavelength. Thus, in order to avoid damage to data, an optical fiber of a transmission line or an optical transmission device which transmits and receives a WDM signal through the transmission line puts in redundancy so as to be substituted with a backup and saved if a failure occurs.
An optical transmission device provided to an optical transmission system such as a node of a WDM network has a transponder (wavelength transformer) for every wavelength which transforms an optical signal emitted from a client device (optical transceiver) into a wavelength suited to wavelength division multiplexing, wavelength division multiplexes the optical signal wavelength-transformed by the transponder by using a multiplexer (wavelength division multiplexer), and provides a transmission line with a WDM signal. Further, a WDM signal received from the transmission line is separated for each of the wavelengths, and the separated signals are respectively wavelength-transformed by the transponder, and then provided to the client device.
It is generally known as a redundant configuration of such an optical transmission device to provide a backup transponder extra in addition to a plurality of transponders provided for respective wavelengths, e.g., as disclosed in Japanese Laid-open Patent Publication No. 10-210008.
Redundancy of the optical transmission device having such a redundant configuration, however, depends on the number of backup transponders. That is, if just one backup transponder is provided, one wavelength only can be substituted and be saved, and failures occurring to a plurality of transponders cannot be dealt with. If redundancy is under consideration and more backup transponders are provided, however, equipment cost resultantly increases.
Thus, it is disclosed in Japanese Laid-open Patent Publication No. 2006-166037 that an optical transmission device is provided with an optical switch unit configured to change a connection path between a transponder and a client device so as to substitute and save an optical signal in failure by switching of the optical switch unit. That is a substitute-and-save technology for stopping transmission of an optical signal of a lower priority than an optical signal in failure, and switching the optical switch unit so that the transponder of the stopped optical signal is used for transmission of the optical signal in failure.
According to the technology disclosed in Japanese Laid-open Patent Publication No. 2006-166037, as an own station cannot share information as to switching of the switch unit with an opposite station, each of the stations cannot know in which priority order the transponder that the other station uses as a substitute is ranked. Thus, the stations need to make an arrangement with each other in advance as to substitute order such as from which transponder each of the stations uses as a substitute.
In this case, e.g., if failures occur on optical signals of the first and second priority orders at the same time, which of the fifth priority and the sixth priority is assigned to which of the first priority and the second priority as a substitute may possibly disagree between the two stations.